1. Field of the Invention
The present invention relates to an image forming apparatus and an optical scanning device. In particular, the invention relates to an image forming apparatus having an optical scanning device for emitting a light beam and scanning a surface of a photosensitive member with the light beam (exposing the surface to the light beam) as well as to the optical scanning device provided in the image forming apparatus.
2. Description of the Related Art
In electrophotographic image forming apparatus such as laser printers, a light beam that is emitted from a light emitting unit including a light source such as a semiconductor laser is deflected by a deflector such as a polygon mirror and a uniformly charged surface of a photosensitive member is scanned with (exposed to) the light beam to form a latent image. The latent image is visualized with toner and the toner image is transferred to a recording medium such as a recording sheet, whereby an image is formed on the recording medium.
As for a scanning device for emitting a light beam and scanning the photosensitive member surface with the light beam (exposing the photosensitive member surface to the light beam), it is necessary to assemble the light emitting unit, the polygon mirror, the scanning optical system for guiding a light beam to the photosensitive member, and other components in such a manner that they are positioned with high accuracy. Therefore, in many cases, the scanning device is manufactured as an optical scanning device in which the components are mounted in a resin-molded frame.
On the other hand, to a user who uses a printer in his or her home, it is convenient if a printer can be installed in a space that is not very wide, for example, on a desk. Therefore, the demand for the miniaturization of image forming apparatus such as laser printers is great. However, if lenses as components of the scanning optical system are reduced in height to miniaturize the optical scanning device, it is necessary to adjust the position of an emitted light beam more accurately.
JP-A-2001-108922 discloses an exemplary method for adjusting the position (in particular, the position in the sub-scanning direction that is perpendicular to the scanning direction of the deflector such as a polygon mirror) of a light beam.
In general, in such optical scanning device, a laser beam emitted from a light source is deflected for scanning in the main scanning direction by a polygon mirror and then guided to the surface of the photosensitive drum by lenses and mirrors. To adjust the write start position of an image on the photosensitive drum, such optical scanning devices are also equipped with a detecting unit for detecting, to produce a sync signal, part of a laser beam that has been deflected for scanning by the polygon mirror.
To miniaturize or reduce the cost of an optical scanning device, it is preferable that the detecting unit be small in size. However, since the light-receiving surface of a small detecting unit is also small, there may occur a case that a laser beam does not shine on the light-receiving surface properly because of influence of mounting errors of mirrors, etc., for guiding the laser beam to the detecting unit.
In view of the above, an image forming apparatus having the following configuration was proposed in which the position of the detecting unit can be adjusted so that a laser beam shines on the detecting unit properly.
That is, a support plate that holds a BD (beam detector) unit in which a BD sensor as the detecting unit, a BD lens for condensing a laser beam onto the BD sensor, and a slit plate for reducing the light quantity to prevent saturation of the BD sensor are unitized is fixed to a side plate of the main body in such a manner that its position can be adjusted. More specifically, the main body side plate is formed with a pair of embossed slide guides and the support plate that holds the BD unit is formed with long holes in which the respective embossed slide guides of the main body side plate are inserted. After the mounting position of the BD unit is adjusted so that a laser beam shines on the center of the light-receiving portion of the BD unit by moving the support plate vertically together with the BD unit, the support plate is screwed to the main body side plate (refer to JP-A-2000-255096).
The above configuration enables, in initial assembling, an adjustment for causing a laser beam to shine on the BD sensor properly.